In huge fermenting vat of yeast
Yeast is a single celled organism and it's essential to the beer making process. Yeast consumes simple sugars and metabolizes them into alcohol and carbon dioxide. Without yeast, beer would not have its fizz or its buzz.
Photo credit: Photographs for Fortune by Berenice Abbott
Original image and text source: Fortune, June 1938
A special tribe of yeast plants changes the wort into Ballantine's Ale
Image source: Beer—Industry, The circulating collection of the Print and Picture Collection, Free Library of Philadelphia